Selective photothermolysis is a method, described by Anderson and Parrish in 1983 (“Selective Photothermolysis: Precise Microsurgery by Selective Absorption of Pulsed Radiation”, Science, Vol. 220, pp. 524-527), for destroying certain diseased or unsightly tissue, on or near the skin, with minimal damage to the surrounding healthy tissue. The tissue to be destroyed is generally characterized by significantly greater optical absorption at some wavelength of electromagnetic radiation than the surrounding tissue. The prior art methods include irradiating the target and the surrounding tissue with pulsed electromagnetic radiation, usually visible radiation, that is preferentially absorbed by the target. The energy and duration of the pulses is such that the target is heated to between about 70° C. and about 80° C., at which temperature the proteins of the target coagulate. Because the target absorbs the incident radiation much more strongly than the surrounding tissue, the surrounding tissue absorbs much less heat and for, at least short time periods of exposure, does not reach a temperature to cause damage. However, the surrounding healthy tissue must be prevented from heating up over extended heating period. Extended heating periods are required for larger skin disorders such as varicose veins.
Usually, the radiation source is a laser, for example a flashlamp-pulsed dye laser. A laser source has the advantage of being inherently monochromatic. Other sources include broad band sources used in conjunction with narrow band filters, as described, for example, by Gustaffson in Patent publication. WO 91/15264. A similar device, called the “Photoderm-VL”, is manufactured by ESC Medical Systems.
Suitable targets for selective photothermolysis include birthmarks, port-wine stains, spider veins, and varicose veins, all of which tend to be much redder than the surrounding tissue because of their higher concentration of oxyhemoglobin-containing red blood cells.
Anderson and Parrish used light of a wavelength of 577 nanometers, corresponding to the 577 nanometer oxyhemoglobin absorption band. It was subsequently determined (Tian, Morrison, and Kurban, “585 nm for the Treatment of Port-Wine Stains”, Plastic and Reconstructive Surgery, vol. 86 no. 6 pp. 1112-1117) that 585 nanometers is a more effective wavelength to use.
One constraint on the pulse duration is that the surrounding tissue must not be heated to the point that it, too, begins to coagulate. As the disorder (hereinafter sometimes referred to as the “target”) is heated, heat is conveyed by convection and conduction from the target to the cooler surrounding tissue. To keep the surrounding tissue from being heated to the point of damage, the pulse length in the prior art is kept on the order of the target's thermal relaxation time. For relatively small targets, such as birthmarks, port-wine stains, and spider veins, typical pulse lengths are on the order of hundreds of microseconds. For varicose veins, pulse lengths on the order of milliseconds should be used.
A complication arises in the treatment of varicose veins by selective photothermolysis. The normal tissue surrounding varicose veins typically includes other blood vessels, notably capillaries, that also absorb the incident radiation but, being much smaller than the varicose veins, have much shorter thermal relaxation times. Therefore, heat diffusing from these other blood vessels tends to heat the surrounding tissue to the point of damage to the surrounding tissue and/or to those other blood vessels. The damage to the surrounding tissue may include scarring.
Recently, selective photothermolysis also has been used to treat psoriatic skin tissue. Psoriasis is a non-contagious skin disorder that most commonly appears as inflamed swollen skin lesions covered with silvery white scale. This most common type of psoriasis is called “plaque psoriasis”.
Psoriasis comes in many different variations and degrees of severity. Different types of psoriasis display characteristics such as pus-like blisters (pustular psoriasis), severe sloughing of the skin (erythrodermic psoriasis), drop-like dots (guttate psoriasis) and smooth inflamed legions (inverse psoriasis). The degrees of severity of psoriasis are divided into three important categories: mild, moderate and severe.
Skin cells are programmed to follow two possible programs: normal growth or wound healing. In a normal growth pattern, skin cells are created in the basal cell layer, and then move up through the epidermis to the stratum corneum, the outermost layer of the skin. Dead cells are shed from the skin at about the same rate as new cells are produced, maintaining a balance. This normal process takes about 28 days from cell birth to death.
When skin is wounded, a wound healing program, also known as regenerative maturation, is triggered. Cells are produced at a much faster rate, theoretically to replace and repair the wound. There is also an increased blood supply and localized inflammation. In many ways, psoriatic skin is similar to skin healing from a wound or reacting to a stimulus such as infection.
Lesional psoriasis is characterized by cell growth in the alternate growth program. Although there is no wound at a psoriatic lesion, skin cells, also referred to as keratinocytes, behave as if there is. These keratinocytes switch from the normal growth program to regenerative maturation. Cells are created and pushed to the surface in as little as 2-4 days, and the skin cannot shed the cells fast enough. The excessive skin cells build up and form elevated, scaly lesions. The white scale (called “plaque”) that usually covers the lesion is composed of dead skin cells, and the redness of the lesion is caused by increased blood supply to the area of rapidly dividing skin cells.
Flash-lamp-pumped pulsed dye laser beams have been used to selectively destroy cutaneous blood vessels. Light passing through the epidermis is preferentially absorbed by hemoglobin, the major chromophore in the blood of the ectatic capillaries of the upper dermis. The radiant energy is converted to heat causing the thermal damage and necrosis in the target. Flash-lamp-pumped pulsed dye laser in general destroy the targeted dermal disorder. The problem is the prevention of damage to the surrounding healthy tissue.
For example, port wine stains are known to be characterized by normal epidermis overlying an abnormal plexus of dilated blood vessels located on a layer in the upper dermis. The predominate endogenous and/or cutaneous chromophores that absorb light at the 585 nanometer wavelength produced by flash-lamp-pumped pulsed dye laser are melanin and hemoglobin. Accordingly, the overlying epidermal pigment layer acts as an optical shield through which the light must pass to reach the underlying lesion such as those caused by port wine stain blood vessels. The absorption of laser energy by the melanin causes localized heating in the epidermis and reduces the light dosage reaching the target thereby decreasing the quantity of heat in the targeted area, leading to sub-optimal blanching of the tissue disorder or necessitating increased time periods of treatment with consequent increased risk of healthy tissue damage, unless steps are taken to protect the healthy tissue.
In the past various cooling methods, ranging from applying ice to the epidermis to spraying the epidermis with a cryogenic gas were used to assure that necrosis due to the applied radiant energy only effects the target and not the surrounding or overlying tissue. See, for example, the article by A. J. Welch et al entitled “Evaluation of Cooling Techniques for the Protection of the Epidermis During ND-YAG Laser Radiation of Skin Neodymium-YAG Laser in Medicine”, Stephan N. Joffe, editor (1983). Skin cooling with freon proved effective in less than 30% of the cases, according to the article. A patent showing skin cooling with cryogen is U.S. Pat. No. 5,814,040, the disclosure of which is hereby incorporated herein by reference.
Other prior art cooling methods used to prevent damage to healthy tissue include the use of lens-like contact devices having high thermal conductivity and having a refractive index that enables the optical radiation to be coupled with the epidermis i.e., a refractive index of approximately 1.55. Thus, the contact device is preferably formed of a high density material such as sapphire or other similar optically transparent glass or plastic. See, for example, U.S. Pat. No. 5,595,568, the disclosure of which is hereby incorporated herein by reference.
Accordingly, since skin cooling depends on many uncontrollable factors, those in the art of selective photothermolysis are still searching for systems that destroy the targeted tissue disorder without damaging overlying and surrounding healthy tissue.